Copper staining issues. I have a in ground fiber glass pool that holds 7000 gallons. I can remove the stain using Stain Out product which works great. I vac the pool out after about an hour or so circulation. Then a I add a bottle of Metal Free to the pool, this is recommended from a pool chemical supplier. Pool looks great once this all complete. Stain returns two to three days later. I am dumping a lot of $$$ that I dont have that kind of $$ to wasting. We did add a heater this year, but i have had this issue in the past, but once I have done the above steps, It is gone. What can I do to the copper out. My chemicals PH & Chlorine levels are low right now. Please help.